Im not sure if you all know what a coast down test is but its a simple test that allows you to calculate the drag coefficient and the rolling resistance of a car.

Im doing this for a class project i have a 2nd gen rx7 and im going to compare the 1st 2nd and 3rd gen models. What i need from you guys is if any of you can help me get some data for 1rd gens. Heres how the test goes

this test has to be done on a relatively calm day (no wind) and a level road

you need to get your car up to 40 mph and then put it in neutral and time how long it takes to drop to 35 mph
then do the same from 15 mph to 10 mph. thats all.

if a couple of you guys can get me some times that would be great. thanks in advance, i'll post my paper for you guys to read when im done

and if anyone knows the frontal area for a first gen that would be great too.

 

Isnt this highly dependant on the condition of the vehicle? I mean, the different models have different accessories, meaning more or less weight, and the condition of the bearings, brakes and other things can greatly affect the measurements. So a 3rd gen could be better simply because the bearings are newer... Im not trying to rain on your parade, just trying to point out factors that could offset your data because if you are trying to measure the drag of the car, and the breakes are dragging, your readings are gonna be wrong. Of course, Im sure you know all that already .

~T.J.

 

there are prob lots of factors but this is a real test and it seems to work cause its used in industry all the time.

 

please somoene help!

 

I'd like to help you out, but my car isn't close to running yet and I doubt I can push it 40mph.

 

thanks though

 

I'm guessing if he can get like 10 different times he'll get a rough average for an SA/FB.

For what it's worth I'll give it a shot next time I'm on some level road. There's not that much level road around here! I remember reading something about how impressive the drag coefficient is for the SA/FB given when it was designed. I'd like to know roughly what it is.

..Matt

 

if you do a search its all there out there.. sumwhere.. heh

 

thanks cortex, let me know your times asap !

 

Actually bbp, do you mind if I change the speeds a little. My speedo is in km/h as I'm in New Zealand.

I converted the mph to km/h and it goes like this..

64km/h -> 56km/h
then
24km/h -> 16km/h

How badly is it going to stuff you up if I do this:

65km/h -> 55 km/h
then
25km/h -> 15km/h

If that doesn't suit let me know some km/h values that are multiples of 5... Judging 1km/h on the speedo is a little hard!

..Matt

 

i guess if you convert the mph to kph it should be ok, and if you round up to something easier it should fine too. I'll fudge some numbers thx

 

My car (83 GSL with mods in the sig, and weighing 2100 pounds):

40 to 35: 8 Seconds
15 to 10: 13 Seconds

Hope that helps. The 15-10 was on a slight downhill slope, so it might be a little high, and I may do it again if I find another appropriate spot. Anyway, as I said before, I hope that helps .

~T.J.

 

awesome thanks tj!

 

Drag coefficient of the original series 1 1978 RX-7 in the wind tunnel was 0.36, or 0.35 with factory rear spoiler fitted.

The Cd was reduced to 0.34 in 1981 when the front bumper was redesigned with polyurethane and intergrated front spoiler. The equated to 5hp increase.

I believe the Cd was marginally lower on the 84/85 model due to the redesigned front apron but I can't find the source.

Any front air dam or ridiculous wing will of course increase the Cd and hinder performance.

Anything close to 0.3 Cd is VERY good, so for essentially a 70s body, the RX-7 is great aerodynamically. The FD as a cmoparison has around 0.29 depending on the year and the spoiler fitted.